These devices comprise means for holding back a sliver of fibres and a fibre-separating device capable of separating the fibres of the sliver from one another on leaving said means for holding back the sliver, this fibre-separating device being arranged in an enclosure at a reduced pressure, connected through a suitable conduit to inlet of the spinning unit, for example of the rotor type. The part played by the fibre-separating device is the changing of a sliver of fibres more or less adhering together into fibres separated from one another which are flung into the flow of air intended to carry them into the spinning unit.
Up until now, two types of fibre-separating devices have been used, namely; the fibre separator with doffer and the fibre separator with drafting.
In the fibre separator with doffer, the sliver of fibres is held back by a device comprising a feed roller controlling the feed of fibres, the roller being combed by a procupine drum which separates the mass of fibres not held back by the feed device in order to throw them, by centrifugal and pneumatic effects, into the flow of air which flows towards the spinning unit.
In the fibre separator with drafting, there is ensured by a very high rate of drafting, hackling of the sliver in such a way that the latter enters into the flow of air with a structure such that the cohesion between the fibres is sufficiently weak for the fibres to be detachable from one another when they reach the flow of air which carries them along into the spinning unit.
Now, in the actual state of the art, it is considered that neither one of these two types of fibre separator gives complete satisfaction. In fact, the combing drum is more efficient than the fibre-separator with drafting for the working of the short fibres and this advantage is all the more marked the shorter are the fibres, this because of the extended division of the mass of fibres supplied. On the contrary, for the fibres of a length of from 40 to 60 mm and more, the fibres are worked by drafting in a manner less coarse than by combing, which reduces to the minimum the breaking of fibres due to the positive extraction of the fibres by the drafting rollers. On the other hand, in the fibre separator with drafing, a high pressure arises in the vicinity of the drafting rollers, especially if the latter rotate at high speed, which is often sought after, and this high pressure brings about a detrimental lateral spreading of the fibres of which part thus breaks free of the rollers.